Human History Written in Stone and Blood

Demographic Expansions …

What, then, might have triggered the onset and termination of these two bursts of creativity and ingenuity? The answer may lie in the history of expansions and isolations of hunter-gatherer populations, as revealed by recent studies of mitochondrial DNA (mtDNA) in living humans. These genetic sequences are inherited maternally, meaning they do not recombine with paternal-source DNA generation after generation the same way nuclear DNA does. Mitochondrial DNA records the pattern of ancient population expansions, declines and isolations, albeit accompanied by a number of assumptions and complications that still thwart consensus on the demographic history of
H. sapiens
. Still, for sub-Saharan Africa, new genetic data offer a much clearer picture of the major demographic events that affected the diversity and dispersal of our species.

One of the most recent studies, by Quentin Atkinson of the University of Oxford and his colleagues, examined four major mtDNA lineages indigenous to Africa—the L0, L1, L2 and L3 groups of haplotypes. The L3 haplogroup is the youngest and is especially interesting because all mtDNA lineages found outside of Africa derive from just two haplogroups (M and N) that descend from L3. Intriguingly, when the timing and pattern of expansion of the four major haplogroups were estimated for the last 150,000 years, the first marked increase in population size began some time between 86,000 and 61,000 years ago in the L3 haplogroup—a time span that includes the Still Bay and Howieson’s Poort. No other haplogroup expanded substantially until more recently than 20,000 years ago.

It is tempting to link the population explosion of the L3 haplogroup with significant environmental events that occurred about 75,000 years ago. As recently reported by Christopher Scholz of Syracuse University and his colleagues, 60,000 years of severe drought in East Africa ended around that time, followed by the onset of generally wetter and more stable conditions. Also, the supereruption of the Toba volcano on the island of Sumatra—the largest eruption known to have occurred on Earth since the emergence of the genus
Homo
—occurred about 74,000 years ago. The injection of volcanic ash and aerosols into the atmosphere is thought by some scientists to have triggered a brief, so-called volcanic winter around the world that reduced the size of human populations. But the breaking of East African megadroughts cannot account for the demographic expansion of only one of the four haplogroups. And the Toba supereruption is incompatible with the observed increase in size of the L3 haplogroup. As a result, Atkinson and his colleagues have dismissed environmental change as the sole factor responsible for the rapid growth in L3 populations and the migration of their descendents out of Africa 10,000 years later.

As an alternative explanation, they proposed that some form of cultural innovation and increase in behavioral complexity by members of the L3 haplogroup gave them a competitive edge over rivals, perhaps by improving technological efficiency, economic productivity, social cohesion and group coordination. Our new ages for the Still Bay and Howieson’s Poort industries place them squarely in the same time frame as these estimated human population expansions. We cannot say whether their innovative attributes were a cause or a consequence of the L3 expansion. But the latter would seem more likely, given the thousands of miles separating southern Africa from East Africa, where the L3 haplogroup is most diverse and is thought to have originated. But resolving cause and effect here requires a fine-tuned chronology for the L3 expansion, so that its precise timing can be compared with the start and end dates of the Still Bay, the Howieson’s Poort, and the similarly sophisticated stone toolkits in North and East Africa.